Abstract This is an application for a R03 award for Dr. Baback Roshanravan, acting instructor at the University of Washington. Dr. Roshanravan is in his fourth year of a NIH K23 investigating the impact of chronic kidney disease (CKD) on skeletal muscle mitochondrial function. His research focus is the application of non-invasive, dynamic in vivo Magnetic Resonance Spectroscopy and Optical Spectroscopy (MRS/OS) to investigate muscle mitochondrial metabolism and its association with physical functioning. Findings from his current study have demonstrated substantial impairments in leg muscle mitochondrial metabolism and muscle endurance in patients with CKD. These findings motivated the proposed R03 that will add ex-vivo high-resolution respirometry using human leg muscle biopsy tissue and live peripheral blood monocyte (PBMC) to his ongoing studies. These studies will be nested within a NIH funded trial of exercise on muscle mitochondrial function among patients with CKD during the applicant's K23 award. To achieve these objectives he is collaborating with Dr. David Marcinek, an expert in mitochondrial physiology and skeletal muscle function. Sarcopenia (skeletal muscle impairment) is a common and debilitating complication of CKD associated with exercise intolerance and poor physical performance leading to mobility limitation and loss of functional independence. Impaired muscle and systemic mitochondrial metabolism are central candidate mechanism of skeletal muscle impairment and poor physical performance in CKD. We have shown that patients with CKD suffer decreased efficiency of muscle mitochondrial ATP generation per unit of oxygen consumption (P/O ratio) with application of in vivo MRS/OS. Reduced P/O is an early marker of redox stress associated with activation of muscle proteolysis and fatigue. These findings suggest endogenous mitochondrial reactive oxygen species (mtROS) play a central role in muscle fatigue. Increase systemic ROS may also increase vulnerability of multi- system mitochondria to physiologic stress contributing to impairments in integrated physical performance measures (mobility). Specifically bioenergetic profiling of live PBMC can help measure mitochondrial reserve capacity (RC) that captures the multi-system burden of ROS and inflammation on integrative measures of physical performance. In this study we will employ ex-vivo methods to dissect mitochondrial physiology and identify mechanisms underlying mitochondrial bioenergetics deficits in patients with CKD not treated with dialysis. First we will test if CKD is associate with mitochondrial dysfunction defined as greater production of mtROS in permeabilized muscle fibers (PMF) and diminished bioenergetic reserve capacity (RC) in live monocyte (PBMC). Second we will describe the association of these measures with muscle fatigue and mobility (gait speed). Third, we will test if mtROS in PMF and RC in PBMC are improved with exercise. If successful, these experiments will help identify novel therapeutic targets to guide future pharmacologic therapies to improve physical limitations in CKD.